For the most part the polymerization or curing of monomers to high molecular weight or highly crosslinked polymers by means of high energy electron beam radiation has been limited to a few general types of monomers. Monomers and oligomers containing acrylate and methacrylate groups have been extensively used as electron beam curable coatings and adhesives. Monomers and oligomers containing carbon-carbon double bonds, such as vinyls, allenes, and dienes, are also known to be electron beam curable. These monomers and oligomers react by means of free radical mechanisms and under high energy ionizing radiation such as electron beam require no free radical initiators. However, these resins tend to be toxic and expensive. Of greater concern is the fact that the polymers produced by crosslinking these materials are of limited value for many applications. Such drawbacks include low glass transition temperature (or low service temperature), low mechanical properties such as strength and modulus, and high shrinkage when cured.
What is desired is a means by which higher performance resins can be processed at any temperature, cured by means of electron beam radiation, and still achieve the same thermal and mechanical properties as when thermally cured. A recent patent by Aerospatiale (Eur. Pat. Appl EP 499,542) described a process by which bismaleimides can be cured at ambient temperature by electron beam radiation to yield materials with good mechanical properties and glass transition temperatures in excess of 300.degree. C. Coreactive diluents were added to the bismaleimides to make them processable at lower temperatures.
Epoxy resins are much more widely used than bismaleimides although their thermal properties are not quite as high. Epoxies are produced in hundreds of millions of pounds annually and are widely used in a variety of applications (L. V. McAdams and J. A. Gannon, "Epoxy Resins", Encyclopedia of Polymer Science and Technology, John Wiley & Sons). A method of curing epoxy resins by means of electron beam radiation would therefore be of considerable value. Visible-ultraviolet curing of thin films of cycloaliphatic epoxies using a cationic photoinitiator is well known (J. V. Crivello, Advances in Polymer Science, Vol. 61, p. 1, 1984).
However, these processes are limited to thin coatings due to the low penetration of visible-ultraviolet radiation. Thermal and mechanical properties of these materials are low and curing is incomplete. There has been little reported success with the electron beam curing of epoxies. Crivello (J. V. Crivello, M. Fan, and D. Bi:, Journal of Applied Polymer Science, Vol. 44, p.9, 1992; U.S. Pat. No. 5,260,349) has recently disclosed a series of highly reactive cycloaliphatic silicone epoxies which are electron beam curable in the presence of certain cationic photoinitiators (diaryliodonium and triarylsulfonium salts). Crivello further reported that cycloaliphatic epoxies and conventional bisphenol A based epoxies cured poorly under the same conditions. Massive doses of electron beam radiation were required. Others (R. S Davidson and S. A. Wilkinson, Journal of Photochemistry and Photobioiogy A:Chemistry, Vol. 58, p.123, 1991) have also reported that examples of these same types of photoinitiators do not lead to complete electron beam curing of certain cycloaliphatic epoxies. It would appear that in some cases cationic photoinitiators react upon exposure to high energy electron beam radiation to produce cationic species which are capable of propagating cationic polymerization. However, degree of curing varies considerably.